Silk-screen printing is a printing process which uses a form—referred to as frame or sieve—constituted by a fabric with a very fine mesh, which is left permeable to the ink in the areas of image to be reproduced and impermeable in the other areas.
The frame is formed by a network that can be made with threads of polyester, nylon or steel stretched over iron or aluminium frames of variable size at a tension of 4-7 bar with a more or less fine mesh according to the chromatic effect that it is desired to obtain, and bears the form of the pattern to be reproduced. Frames normally used for silk-screen printing have a woof that can range from 10 to 150 threads/cm2 according to the type of ink or paint product used and the amount thereof to be released onto the substrate.
Preparation of the frame can be performed manually by tracing thereon with a lithographic crayon the pattern that it is intended to reproduce. This closes with its trace the underlying layers between the woof and the warp. Next, with a special glue, all the spaces not involved in the pattern are closed, and then, using a solvent (alcohol or benzene), all of the thick trace of the crayon is removed, exposing the holes that had been occluded. Preparation of the frame can also be performed mechanically, in particular photomechanically.
Printing may be manual or carried out using special machines. In the first case, the ink is distributed using a spatula, referred to as doctor blade or doctor knife, over the sieve stretched over the appropriate frame.
The doctor blade, in fact, by applying pressure as it slides from one end to the other of the frame causes passage through the network of the ink on the substrate that is to be printed. The average thickness of dry ink deposited for each pass is between 50 and 100 μm.
The machinery used for silk-screen printing is substantially made up of surfaces that apply suction pressure and/or that use special glues either in line or rotating on a carousel with one to eight colours and intermediate drying hoods with IR or UV lamps or hot-air lamps. Said drying devices may moreover be equipped with travelling bands or belts of the length necessary for polymerization of the ink of the paint product used or can carry out manual drying on frames, which can be set on top of one another. This machinery may moreover be equipped with a sheet-in/sheet-out device.
The silk-screen printing system is used generally for printing of fabrics, packaging made of paper, aluminium or plastic materials, road signs and also in the decoration of furniture, crockery and toys.
Printing inks are generally formed by a dyeing part and by a binding part. The dyeing part can be obtained with pigments and with soluble dyes, where by “pigment” is meant an insoluble coloured compound in the binder of the ink, whilst by “soluble dye” is meant a coloured compound that is soluble in the binder. In turn, the binder is constituted by a vehicle, a modifier of the vehicle, and a solvent.
The typical composition of an ink is given in Table 1.
TABLE 1Dyeing partBinding partPigmentSoluble dyeVehicleModifierSolventOrganic(transparent)NaturalPlastifiersAliphatic(transparent)resinsWaxescompoundsInorganicSyntheticOilsAromatic(covering)resinsWetting agentscompoundsStabilizing agentsKetonesAnti-foamingEstersagentsWaterDrying agents
The dyeing part determines the chromatic characteristics of the ink. The organic pigments are characterized by a good transparency, with the exception of carbon black, which is semi-covering, and fluorescent pigments which are covering.
The inorganic pigments possess high covering-power characteristics with the exception of some colours that are transparent (iron blue) or semi-covering (china clay, calcium carbonate, aluminium hydrate). The soluble dyes are, instead, characterized by a very high transparency.
The intensity of the colours is adjusted with covering or transparent thick white inks.
The binding part of a printing ink is constituted, as has been seen, by a vehicle, a modifier, and a solvent.
The vehicle is constituted basically by natural or synthetic resins that bestow upon the ink fundamental characteristics from the chemico-physical standpoint.
Natural resins have had a considerable importance in the formulation of printing inks. Currently, the tendency is, however, to replace them with resins of a synthetic type.
Synthetic resins can be used by themselves or mixed together, and, in a few cases, also mixed with natural resins, should there exist a good degree of compatibility, where by “compatibility” is meant the possibility for two or more resins to form a homogeneous and stable solution that does not give rise over time to phenomena of separation, precipitation, or clouding.
The main resins used are resins of an epoxy, vinyl, hydrocarbon, nitro-cellulose, maleic, melamine, phenol-formaldehyde, urea-formaldehyde, alkyd, polyester, acrylate, or polyurethane type.
Modifiers are used to bestow upon the inks particular characteristics, such as for example plasticity, flexibility, mechanical surface resistance, etc. Esters of phthalic, sebacic and glycolic acids are generally used as plastifiers; as regards waxes, natural ones, such as for example carnauba wax, or synthetic ones, for example polyethylene, are used; as regards oils, linseed, soya and wood oils may be used; finally, as dessicators the salts of manganese, zinc, cobalt and zirconium of naphthene, octoic and linoresinic acids are used.
The solvents have an extremely important dual function: they must bestow upon the ink the fluidity necessary for enabling transfer from the printing form (sieve or frame) to the substrate, and must moreover constitute the mobile phase of the ink-drying process, i.e., the component that must be removed either by penetration or by evaporation or by selective filtration at the moment of application of the ink on the substrate.
The film of ink that remains on the material has a constant thickness, in all cases much greater than that of the typographic and lithographic processes; as a result it achieves much greater covering-power effects. This enables prints with high resistance to light and to chemical agents to be obtained. Furthermore, the system does not call for high printing pressures that are necessary in other processes.